Photosynthetic characteristics of marine phytoplankton from contrasting physical environments

Abstract

Two suites of phytoplankton samples have been collected in consecutive years at various times over a day from selected depths within vertically mixed and stratified water columns in the western Irish Sea, in order to provide a range of possible light histories within the populations collected. Values for the maximum rate of 14C retention (Pmax) and the initial slope of the 14C retention: light intensity curve (α) were obtained. Supra-thermocline samples from the stratified water exhibited higher Pmax values than corresponding subthermocline samples. Higher values of α were also generally associated with samples from the supra-thermocline zone of the stratified region. Differences in the depth distribution of Pmax and α in the mixed water were small, except in the presence of a shallow thermocline. In one suite of samples from the stratified water, a diurnal increase in the Pmax values of the supra-thermocline samples was observed. Pmax values obtained from the samples from the mixed water were interpreted in relation to the distribution obtained from the samples from the stratified zone. Data from both the contrasting sites visited for one sample suite demonstrated a two-phase relationship between the chlorophyll a concentration and both Pmax and α. The rates of 14C retention of the first suite of samples were estimated by two techniques. The average differences in the retention were greater in samples from the sub-as opposed to suprathermocline zone. No trends were apparent in the smaples from the mixed waters.